The invention relates to a flexible pipe element for the exhaust system of an internal combustion engine. Such a flexible pipe element comprises an annular corrugated or helically corrugated metallic bellows, with exhaust gas from the internal combustion engine can flow through it. A line section is arranged upstream in reference to the metallic bellows, which is also embodied flexible or inflexible, for example as a smooth tube.
Flexible pipe elements of the present type are installed in the pipe runs of exhaust systems in order to prevent transferring movements of the exhaust pipe runs, connected fixed to the internal combustion engine, to other parts of the exhaust system, which are generally fixed to the vehicle in which the internal combustion engine is installed, or perhaps connected fixed to a building in which a stationary internal combustion engine is located.
A metallic bellows is capable to compensate axial, lateral, and angular motions generated by the temperature-related changes of length, load change motions of the internal combustion engine, as well as the impact shocks occurring in vehicles during the driving motion and the like. Furthermore oscillations of most different frequencies, generated in the internal combustion engine by the moving parts and by pulsing gas flow, are compensated by the metallic bellows and thus not forwarded to the continued parts of the exhaust system to the extent the natural frequency is sufficiently damped. This prevents that the oscillations of the internal combustion engine, radiated from the exhaust system in the form of impact sound, are forwarded to a vehicle body, which otherwise would lead to undesired humming noise inside the vehicle.
Flexible pipe elements of the present type can be absolutely or practically fluid-tight by the metallic bellows used, depending if it has been produced by way of deformation of a tube or by winding a metallic profile. The metallic bellows may be embodied with one or more walls, with in case of a multi-wall embodiment the individual layers of the wall may comprise different materials, for example to promote corrosion resistance.
A flexible pipe element of the present type usually comprises, in addition to the metallic bellows, a metallic tube coaxially arranged inside thereof. This serves to guide the flow of the exhaust gas. Hereby any excessive thermal stress of the metallic bellows by the hot exhaust is prevented. Furthermore any eddying of the exhaust in the corrugations of the metallic bellows is prevented, which on the one hand would increase the flow resistance and on the other hand might cause considerable noise.
Such a metallic tube commonly represents a wound tube comprising metallic profiles or hooked profiles folded into agraffes which ensures the necessary mobility and furthermore acts in an oscillation-damping fashion due to the internal friction of the profiles against each other. If necessary, the oscillation-damping effect of the metallic tube can be reinforced by additional oscillation-damping components, such as a webbed tube pulled over the exterior of the metallic bellows or arranged between the metallic bellows and the metallic tube. Examples of such flexible pipe elements with a metallic bellows, metallic tube, and a webbed tube are also disclosed in EP 0 875 669 A2 and DE 196 41 963 C1.
Flexible pipe elements of the present type, particularly with additionally provided metallic tubes and damping elements, have successfully been used for years particularly in motor vehicles. Here, they are beneficially installed in the proximity of the engine between the exhaust manifold and the catalytic converter, because the closer the flexible line segment is arranged in reference to the rotating axis of the engine the lower the relative motions that need to be compensated by deformation. Accordingly a place of installation near the engine beneficially allows short metallic bellows of flexible line segments. Depending on the local conditions of the place of installation the cross-section is embodied either essentially cylindrical or oval or flattened like a stadium.
In order to reduce emissions of nitrous oxide from internal combustion engines, particularly diesel engines used in motor vehicles, frequently SCR-converters (Selective Catalytic Reduction Converters) are used in the exhaust system. During operation such a SCR-converter requires ammonia as reduction means for nitrous oxide (NOx). The ammonia required is yielded from urea, which is injected in a liquid form upstream in reference to the SCR-converter into the hot exhaust gas flow. In the event of disadvantageous framework conditions (particularly when exhaust temperatures are too low, slow flow speeds, poor mixture of the urea with the exhaust gas flow, overdose of urea, etc.) urea derivatives form particularly in the coldest wall sections of the exhaust flow. They represent fundamental components for forming HWL-precipitations. As soon as these fundamental components and/or urea derivatives come into contact with a wall of the pipe element precipitations or viscous wall films may develop. They are directly dependent on the HWL-processing upstream in reference to the SCR-converter.
However, in many cases it is desired or even necessary in the exhaust system to install a flexible pipe element of the present type upstream in reference to a SCR-converter, while simultaneously the injection of the urea necessary for the SCR-converter occurs still upstream in reference to the flexible line segment. This can lead to disturbing precipitations of urea derivatives developing in the flexible pipe element after extended periods of operation.
Such precipitations are primarily very disadvantageous when they reach or develop in the area between a frequently provided metallic tube and the metallic bellows. Due to the fact that the metallic tube guides the exhaust flow the intermediate space between the metallic tube and the metallic bellows fails to reach temperatures sufficiently hot to dissolve the precipitations using a regeneration process or during operating conditions with particularly hot exhaust gas flow. The precipitations of urea derivatives therefore accumulate over time particularly in this intermediate space until they completely clog it at some point of time. Even in flexible line segments of the present type, in which no component is provided inside the metallic bellows with gas flown through, precipitations of urea derivatives are disadvantageous because they can precipitate at the corrugations of the metallic bellows and here are then not dissolved by hot exhaust flow because the inside of the corrugations in general it is considerably less hot than the exhaust flow. Such precipitations reduce the mobility of the metallic bellows and/or the metallic tube perhaps provided, which compromises the desired features of the flexible pipe element and, upon respective stress, can even lead to damages of the metallic bellows and the line segment then no longer showing the required fluid tightness.